Mark external/wpt/html/dom/reflection-grouping.html as leaky

TBR=kochi@chromium.org

Bug: 780386
Change-Id: If4286a902cde9c33ef04aebc648b9f5ad822601d
Reviewed-on: https://chromium-review.googlesource.com/748462Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513092}

third_party/WebKit/LayoutTests/LeakExpectations

@@ -44,6 +44,7 @@ crbug.com/755625 external/wpt/beacon/beacon-error.window.html [ Leak ]
 # -----------------------------------------------------------------
 crbug.com/762353 external/wpt/beacon/headers/header-referrer-no-referrer.html [ Leak Pass ]
 crbug.com/651742 external/wpt/content-security-policy/connect-src/connect-src-beacon-allowed.sub.html [ Leak Pass ]
+crbug.com/780386 external/wpt/html/dom/reflection-grouping.html [ Leak Pass ]
 
 # -----------------------------------------------------------------
 # Not revert suspected CL as jam@ request, expected to be fixed soon.